Ultra Wideband (UWB) technology is a wireless technology for the transmission of large amounts of digital data as modulated coded impulses over a very wide frequency spectrum with very low power over a short distance. Such pulse based transmission being an alternative to transmitting using a sinusoidal wave which is then turned on or off, to represent the digital states, as employed within today's wireless communication standards and systems such as IEEE 802.11 (Wi-Fi), IEEE 802.15 wireless personal area networks (PANs), IEEE 802.16 (WiMAX), Universal Mobile Telecommunications System (UMTS), Global System for Mobile Communications (GSM), General Packet Radio Service (GPRS), and those accessing the Industrial, Scientific and Medical (ISM) bands, and International Mobile Telecommunications-2000 (IMT-2000).
UWB was formerly known as “pulse radio”, but the Federal Communications Commission (FCC) and the International Telecommunication Union Radiocommunication Sector (ITU-R) currently define UWB in terms of a transmission from an antenna for which the emitted signal bandwidth exceeds the lesser of 500 MHz or 20% of the center frequency. Thus, pulse-based systems where each transmitted pulse occupies the full UWB bandwidth or an aggregate of at least 500 MHz of narrow-band carrier; for example, orthogonal frequency-division multiplexing (OFDM) can gain access to the UWB spectrum under the rules. Pulse repetition rates may be either low or very high. Pulse-based UWB radars and imaging systems tend to use low repetition rates (typically in the range of 1 to 100 megapulses per second). On the other hand, communications systems favor high repetition rates (typically in the range of one to two gigapulses per second), thus enabling short-range gigabit-per-second communications systems. As each pulse in a pulse-based UWB system occupies a large bandwidth, even the entire UWB bandwidth, such systems are relatively immune to multipath fading but not intersymbol interference, unlike carrier modulation based systems which are subject to both deep fading and intersymbol interference (ISI).
Pulse based wireless communication has come a long way since being first allowed by the Federal Communication Commission (FCC). Able to offer either high data rates or very energy efficient transmissions over short ranges, multiple techniques have been developed for ultra-wideband (UWB) communication including multi-band orthogonal frequency division multiplexing (MB-OFDM), impulse radio (IR-UWB) and frequency modulation (FM-UWB) each with its specific strengths. The potential for very low power communications and precise ranging has seen the inclusion of UWB radios in multiple standards aimed for different applications like low-rate wireless personal area networks (WPAN) with IEEE 802.15.4a and more recently wireless body area networks (WBAN) with IEEE 802.15.6.
UWB systems are well-suited to short-distance applications in a variety of environments, such as depicted in FIG. 1 including peripheral and device interconnections, as exemplified by first residential environment 110, sensor networks, as exemplified by second residential environment 120, control and communications, as exemplified by industrial environment 130, medical systems, as exemplified by medical imaging 150, and personal area networks (PAN), as exemplified by PAN 140. Due to low emission levels permitted by regulatory agencies such UWB systems tend to be short-range indoor applications but it would be evident that a variety of other applications may be considered where such regulatory restrictions are relaxed and/or not present addressing military and civilian requirements for communications between individuals, electronic devices, control centers, and electronic systems for example.
Due to the short duration of UWB pulses in principle it is easier to engineer high data rates and data rate may be exchanged for range in many instances by aggregating pulse energy per data bit, with the appropriate integration or coding techniques. In addition UWB supports real-time location systems and tracking (using distance measurements between radios and precision time-of-arrival-based localization approaches) which in addition to its precision capabilities and low power make it well-suited for radio-frequency-sensitive environments, such as many medical environments. An additional feature of UWB is its short broadcast time.
When considering many applications, such as wireless sensor networks and portable electronics, UWB transceivers should ideally be functionally highly integrated for low footprint, support low cost and high volume manufacturing, and be energy efficient in order to run on a limited power source, e.g. a battery, indoor solar cell, small outdoor solar cell, or those developed upon evolving technologies such as thermal gradients, fluid flow, small fuel cells, piezoelectric energy harvesters, micromachined batteries, and power over optical fiber. UWB has been considered for a long time a promising technology for these applications. By using discrete pulses as modulation, it is possible to implement efficient duty cycling scheme while the transmitter is not active, see for example Hamdi et al in “A Low-Power OOK Ultra-Wideband Receiver with Power Cycling” (Proc. IEEE New Circuits and Systems Conference 2011, pp. 430-433), which can be further improved by using an On-Off Shift Keying (OOK) modulation. Further, some UWB operation frequencies, between 3.1 GHz and 10.6 GHz for example as approved by FCC for indoor UWB communication systems, see for example “First Report and Order in the Matter of Revision of Part 15 of the Commission's Rules Regarding Ultra-Wideband Transmission Systems (FCC, ET-Docket 98-153, FCC 02-48), allow for small antennas which can easily be integrated into an overall reduced footprint sensor solution.
In order to generate very short impulses which conform to a power spectrum density (PSD) mask, multiple approaches have been attempted within the prior art, each of which has different strengths and drawbacks. Most work relates to shaping a short numerical impulse by filtering, see for example Jazairli et al in “An Ultra-Low-Power Frequency-Tunable UWB Pulse Generator using 65 nm CMOS Technology,” (IEEE Int. Conf. on Ultra-Wideband, 2010, pp. 1-4) and Sim et al in “A CMOS UWB Pulse Generator for 6-10 GHz Applications” (IEEE Microwave and Wireless Components Letters, Vol. 19(2), pp. 83-85), or by using an oscillator and a mixer to up-convert the signal, see for example Y. Zheng et al., “A 0.18 μm CMOS 802.15.4a UWB Transceiver for Communication and Localization” (IEEE Int. Solid-State Circuits Conference, 2008, pp. 118-600). However, short impulse filtering requires bulky passive components and generates a fixed pulse pattern whilst mixing uses an oscillator in conjunction with a mixer with high power consumption but does provide spectrum flexibility.
Within low power systems controlling the transmitted PSD is very important to maximize the spectrum utilization by appropriately shaping the pulses. However, in other applications and operating regimes avoiding certain frequency bands may be a requirement in order to reduce noise and the resulting signal interference either to the UWB signal or other signals. For example, global positioning system (GPS) exploit very low power signals, generally within the noise, at 1575.42 MHz, 1227.60 MHz, 1380.05 MHz, 1379.913 MHz, and 1176.45 MHz for the L1 to L5 bands respectively, see for example “On the UWB System Coexistence with GSM 900, UMTS/WCDMA, and GPS” (IEEE J. Sel. Area in Comms., Vol. 20(9), pp. 1712-1721). Whilst mixing can be used for tuning the center frequency of a transmitter, usually along standardized channels as in IEEE standards, such systems generally use pulses with relatively small bandwidths to separate the channels, and apart from skipping certain center frequencies cannot adaptively adjust their spectral utilisation. Whilst good spectral usage and tunability may be achieved with MB-OFDM through the combination of multiple smaller bandwidth channels concurrently such approaches are better suited to high data rate applications due to the increases in transmitter complexity and power usage.
Accordingly, it would be advantageous for an UWB transmitter to exploit an on-demand oscillator in order to up-convert the pulse thereby removing the requirement for a separate mixer. It would be further beneficial for the UWB transmitter to be CMOS logic compatible and for the pulse generation and oscillator to be both digitally tunable in order to provide control over the pulse bandwidth and center frequency and capable of rapid frequency adjustments on the order of the pulse repetition rate (PRR). Such UWB transmitters advantageously, in comparison to MB-OFDM UWB transmitters, providing spectral configurability, by sequentially changing the transmitted spectrum using a frequency and bandwidth hopping scheme. It would be further beneficial for such an UWB transmitter to offer dynamic duty cycling with fast power up time and OOK modulation to provide reduced power consumption by exploiting the low duty cycle of an IR-UWB symbol and the fact that only half the symbols require sending energy.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.